This disclosed subject matter pertains to automated manufacturing environments, such as semiconductor manufacturing, and, more particularly, to a method and apparatus for assigning material transport vehicle idle locations.
Growing technological requirements and the worldwide acceptance of sophisticated electronic devices have created an unprecedented demand for large-scale, complex, integrated circuits. Competition in the semiconductor industry requires that products be designed, manufactured, and marketed in the most efficient manner possible. This requires improvements in fabrication technology to keep pace with the rapid improvements in the electronics industry. Meeting these demands spawns many technological advances in materials and processing equipment and significantly increases the number of integrated circuit designs. These improvements also require effective utilization of computing resources and other highly sophisticated equipment to aid, not only design and fabrication, but also the scheduling, control, and automation of the manufacturing process.
Turning first to fabrication, integrated circuits, or microchips, are manufactured from modern semiconductor devices containing numerous structures or features, typically the size of a few micrometers or less. The features are placed in localized areas of a semiconducting substrate, and are either conductive, non-conductive, or semi-conductive (i.e., rendered conductive in defined areas with dopants). The fabrication process generally involves processing a number of wafers through a series of fabrication tools. Each fabrication tool performs one or more of four basic operations discussed more fully below. The four basic operations are performed in accordance with an overall process to finally produce the finished semiconductor devices.
Integrated circuits are manufactured from wafers of a semiconducting substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises the following four basic operations:                layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced;        patterning, or removing selected portions of added layers;        doping, or placing specific amounts of dopants in selected portions of the wafer through openings in the added layers; and        heat treating, or heating and cooling the materials to produce desired effects in the processed wafer.        
Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process.
To facilitate processing of wafers through a process flow, wafers are typically grouped into lots. Each lot is housed in a common wafer carrier. Carriers are transported to various process and metrology tools throughout the fabrication facility to allow the required processes to be completed to fabricate integrated circuit devices on the wafers.
Modern wafer fabrication facilities employ automated material movement systems to satisfy ergonomic concerns and to maintain a high level of automation. An interbay/intrabay vehicle automated material handling systems may be employed to automate the transfer of wafers to the tools required in the process flow. One factor contributing to the efficiency of the material handling system is the delivery time between tools. Delivery time may vary depending on the distance between tools, the congestion of the tools, and the distance an idle material handling vehicle needs to travel to pick up a waiting wafer carrier.
The transport time between tools is relatively static under normal system conditions, but the time required to locate an idle material handling vehicle, assign it to a lot, and move the assigned vehicle to the current location of the lot (e.g., tool or intermediate storage location) can exhibit significant variability. Hence, the disposition of idle material handling vehicles throughout the fabrication facility impacts the time it takes a completed lot to transit from a current tool to the next required tool, and thus the throughput of the manufacturing system.
The determination of the appropriate parking locations for the material transport vehicles is an aspect that affects the overall efficiency of the system. However, as with other resources in the fabrication facility, the material handling capacity is relatively fixed. In determining a number of material handling vehicles to park in a given controlling area, it is necessary to attempt to provide enough vehicles to respond to transport pickup requests, but also it is not efficient to have too many idle vehicles. An excess number of idle material transport vehicles could result in additional vehicle traffic and extended delivery times, and could also cause an imbalance in the distribution of vehicles across the fabrication facility, further increasing retrieval time variability. Redistributing idle vehicles by making a significant number of parking move requests could degrade system performance by artificially increasing the vehicle utilization by tasking the vehicles with move requests.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.